The Ni-Al intermetallic compounds such as Ni.sub.3 Al(.gamma.') has demonstrated extraordinary properties: high melting point, high ordering energy, thermal hardening, good resistance to oxidation and relatively small density. Those properties make it attractive for aerospatial and structural applications at elevated temperatures.
One of the techniques for forming the Ni-Al intermetallic compounds is obtained by vacuum melting and vacuum casting which is mostly used by the Metals and Ceramics Division of Oak Ridge National Laboratory, U.S.A. Owing to the following suffered disadvantages: a) the metal-crucible interactions are prone to be caused during the melting and casting, b) the cast is apt to have shrinking pores, and c) the cast is too hard to be worked, there are many difficulties encountered according to this technique.
Another technique for forming the Ni-Al intermetallic compound is the powder metallurgy (PM). This technique can permit us to obtain a compound with a relatively high yield rate, accurate dimensions, and a satisfactory microstructure and to easily control the components thereof.
One of the powder metallurgy techniques is the sintering of pre-alloyed powder which mainly processes by the rapid solidification process (RSP), the powder or ribbons by hot isostatic pressing or hot extrusion. Although the final sintered product is of a high density, there is still a key disadvantage: this technique includes too many procedures which have to employ expensive equipment and operate under a protective atmosphere. Besides, the hardness of the pre-alloyed powder is so high that the green forged parts cannot be easily formed and will wear the mold off easily.
Another powder metallurgy technique is the mechanical alloying which processes the pure metal powder in a protective atmosphere by the high-energy ball milling to lower the sintering temperature. According to this technique, some dispersion strengthening particles are added to achieve grain refining and strength increasing. The disadvantages of this technique are, a) the procedure takes too much time, b) the obtained powder is so hard that the pressure for formation is therefore high, c) the ball-milling step causes the pollution problem, and d) the sintering density after the ball milling procedure is lowered.
A further powder metallurgy technique is the reactive sintering. This technique uses the inexpensive elemental metal powder which is softer than the pre-alloyed powder for the initial material, so there are the following advantages: a) the formation thereof is easy to be obtained, b) the sintering temperature can be lowered down to a large scale, and c) the sintering time can be shortened. Whereas, there are also disadvantages: a) the pores are prone to be generated when the reaction heat and the difference of the elemental diffusion rates are high, b) the densification is hard to be obtained, c) the protective atmosphere such as argon, hydrogen, and helium is necessary for preventing the oxidation of aluminum powder, and d) the densified compound is sensitive to the processing parameters such as the heating rate, the interfacial quality, the temperature, and the particle size. It is also noted that if a high density sintered body is to be obtained, a higher heating rate, a finer powder (in .mu.m order) and an externally applied pressure during sintering are all needed, but the equipment that meets the above requirements are extremely expensive.
A further technique for forming the Ni-Al compounds is the chemical technique. The initial material NiCl.sub.2 and AlCl.sub.3 are processed by the co-deposition method to obtain a nickel-aluminum organometallic complex. After a first thermal treatment lower than 1000.degree. C. to burn off the organic function groups to obtain the mixture of Ni.sub.3 C and the non-crystalline Al.sub.2 O.sub.3 and free carbon, and after a second stage of heat treatment above 1300.degree. C. to obtain the intermetallic compound, the Ni.sub.3 Al and NiAl powder whose diameters are below 3 .mu.m are formed. Whereas, this technique whose cost is too high and whose speed is too slow cannot economically meet with the industrial demand.
It is therefore attempted by the Applicant to deal with the above situations encountered by the prior art.